I build Alistar 100% as a tank. Some people go heavy AP, but I feel this is wrong. His primary job is to engage, and single out targets through headbutt and pulverize. He brings utility to team fights through triumphant roar, and his ultimate is (arguably) the best tank ult in the game.
With this build, you'll be focused on getting as much health as possible, becoming a meat shield for your carries. You won't see any 2x-x-x K:D:A ratios. They'll look more like 4:3:20. That just means you're doing your job.

Mastery choice here is pretty straightforward. Since we are going for max tanking, our masteries focus on this. Once we've got 21 in defense, the extra ability power we get helps our Headbutt/Pulverize damage, as the AP to Dmg ratio on our friendly neighborhood steer, is an inviting 1:1.

Rod of Ages gives Alistar every stat he needs and then some. When spamming spells, you'll find that he's very mana-hungry, and this will help to offset that. Acquiring it early gives it time to add growths. If, however, you feel you're controlling your lane *extremely* well, and are getting kills left and right, there's no wrong in going for Leviathan first, so it can absorb those kills.
After these two, we pick up a Sheen. Many Alistars make this their build staple, but I fail to see the utility as much as them. Sure it's nice to boost your attack, but I'd rather live longer to get off -more- auto-attacks, rather than have one boosted up a little so early on. We'll take this for CDR, and give credit toward the advantage its passive gives.
Most tanks opt for mercury treads, but this is unnecessary on Alistar. His ult frees you from CC, making the Mercury Treads downright useless. We take Ionian Boots of Lucidity for the CDR. Boots of Swiftness are not a bad choice either, as you're awfully slow.
To wrap things up, I typically go Frozen Mallet and Sunfire, which should make you downright impossible to kill if you've done well and gotten Leviathan stacks so far. I can see arguments made to switch out one of the last two for a Randuin's Omen-- this is personal preference, honestly.

I max Headbutt soonest, because I feel it is the most vital ability to team ganks, as well as harass. To effectively utilize Pulverize, you need to be in close range to your opponents. This proves difficult in the early game, and pretty much nullifies the ability. However, it's not so tough to charge them, knock them back, and be able to run back before taking a single shot from them.
Headbutt also serves as your bread-and-butter in team gank scenarios. You'll find yourself sneaking around bushes and through backdoors sometimes, simply to Headbutt an enemy to your teammates. With the added damage from Headbutt, they'll be a much easier kill, thus giving you an easier Leviathan stack and furthering the team as a whole.
I tend to ignore Triumphant Roar, simply because it is SUCH a mana drain. In the late game, you'll find it marginally useful, though.

While this may not be the ultra-killer-fantastic build many seek, I'm sure this build serves to create a tank that's hella difficult to bring down. Alistar is one bull that you don't want to grab by the horns!
I am always open to questions/comments/concerns, and welcome your feedback. Thanks for reading, and good luck. :)

*Fun Fact: The name Alistar in olde english also means "protector" a name fitting for one of the game's first tanks.

The mightiest warrior ever to emerge from the minotaur tribes of the Great Barrier, Alistar defended his tribe from all of Valoran's many dangers; that is, until the coming of the Noxian army. Alistar was lured from his village by the machinations of Keiran Darkwill, Baron Darkwill's youngest son, and commander of the Noxian expeditionary force. When Alistar returned home, he found his village burning and his family slain. Bellowing with rage, he charged an entire regiment of Noxus's elite, slaughtering them by the hundreds. Only the intervention of some of Noxus's most skilled summoners checked Alistar's rage. Brought in chains to Noxus, Alistar spent the intervening years as a gladiator pitted in endless battle for the entertainment of Noxus's wealthy leaders. Alistar's once noble soul slowly twisted, and he would have been lost to insanity if not for Ayelia, a young servant girl who befriended him and eventually arranged for his escape. Suddenly free, Alistar joined the newly-formed Institute of War as a champion, hoping to one day visit his final vengeance against Noxus and to find the girl who once again gave him hope.

"If you intend to grab the bull by the horns as a summoner, Alistar might have something to say about that."